Backport PNG/ZIP improvements from PIL repo

- add new FASTOCTREE quantizer with alpha support
- make ZIP compress level and type configurable
- support reading/writing PNGs with paletted alpha
- reduce PNG palette size for images <255 colors

Had to revert 5baa1ac1b8d41fcedce7b12ed1c4a8e87b4851bc (and a fix for that commit b29326b17534f5fec04c0bec35c2f1eff6499f73). Was already handled by the changes.

source https://bitbucket.org/effbot/pil-2009-raclette/commits/3637439d51693c92028a7c4cfa75f5db583de357
https://bitbucket.org/effbot/pil-2009-raclette/commits/5c682473c68352bea416da3e2814081d992f74f8